Angular position sensor

ABSTRACT

A liquid level sensor is provided for use with a container. The sensor protrudes through an opening in the container. The sensor includes a float linked by means of a linkage to a first magnet axially rotatable on a first axis. The first magnet has a magnetic moment with a nonzero component at a right angle to the axis. A divider separates the first magnet from a second magnet having a magnetization and axially rotatable on a second axis. The second magnet has a nonzero magnetic moment at a right angle to the second axis. The first and second magnets are juxtaposed in magnetic linkage so that the second magnet is urged to follow the first magnet in rotation. The divider plugs the opening in the container. A magnetic field sensor is positioned to sense axial magnetic field strength at a location offset from the second axis. Importantly, the magnetization of the second magnet gives rise to a sensed magnetic field at the sensor that is nonsinusoidal with respect to an angle of rotation of the second magnet on the second axis, or gives rise to north and south poles separated by more than a half-circle or axial rotation of the second magnet.

BACKGROUND OF INVENTION

[0001] It is not easy to measure liquid levels, especially where theliquids are hazardous or flammable.

[0002] It is well known in the art to provide a float within acontainer, the float caused to rise and fall by the level of liquid inthe container. The float is linked to a rotating first magnet that iswithin the container, or is at least on the liquid side (inside) of adivider that is joined to an opening in the container. The first magnetis magnetized so that a magnetic moment has a nonzero component at aright angle to the axis of rotation, and preferably its moment isentirely at a right angle to that axis. A second rotating magnet is onthe outside of the divider, and is nearly coaxial with and magneticallycoupled with the first magnet, likewise having a moment (or a componentof the moment) perpendicular to the axis. The second magnet may actuatea pointer providing a human-readable indication of the liquid level. Thesecond magnet is physically nearby to a magnetic flux sensor such as aHall-effect sensor. The flux sensed in the sensor is indicative of theliquid level. The sensed flux signal is converted from analog to digitaland is passed on to other equipment. Mechanisms suitable for use in suchapparatus are described, for example, in U.S. Pat. Nos. 4,987,400,6,041,650, and 6,089,086, incorporated herein by reference.

[0003] Unfortunately, this approach offers many drawbacks. One drawbackis that with the conventional and commonly used magnetization and magnetshape, the sensed flux deviates substantially from linearity withrespect to the actual liquid level. While such nonlinearity can becorrected in software (after the A/D conversion, for example), thisresults in variations in resolution across the range of measuredphysical values such as liquid level, and adds to computational cost.

[0004] Yet another drawback is that with such magnetization and magnetshape, it is impossible to disambiguate certain distinct liquid levelsbased solely on the sensed flux at a particular time; for disambiguationit is necessary to maintain state information such as historicalinformation about recent sensed values. Such disambiguation requiresfrequent data collection and depends upon assumptions regarding howquickly the liquid level might change. The disambiguation problem may beavoided by limiting the permitted angular rotation of the magnets, forexample by choosing the details of the float linkage, such as gearratios. This has the drawback of limiting either the resolution of thesensing system or the dynamic range of the sensing system, or requiringa more expensive analog-to-digital convertor.

[0005] It is thus desirable to provide a system for measurement ofliquid levels or other physical phenomena, which employs a float orother follower linked to a first magnet, and a second magnet linked tothe first magnet, where the electrical output is nearly linear with thephysical phenomenon being measured, and wherein the dynamic range ismaximized and resolution uncompromised, all without expensivepost-processing of data and without expensive high-resolution A/Dconvertors.

SUMMARY OF INVENTION

[0006] A liquid level sensor is provided for use with a container. Thesensor protrudes through an opening in the container. The sensorincludes a float linked by means of a linkage to a first magnet axiallyrotatable on a first axis. The first magnet has a magnetic moment with anonzero component at a right angle to the axis. A divider separates thefirst magnet from a second magnet having a magnetization and axiallyrotatable on a second axis. The second magnet has a nonzero magneticmoment at a right angle to the second axis. The first and second magnetsare juxtaposed in magnetic linkage so that the second magnet is urged tofollow the first magnet in rotation. The divider plugs the opening inthe container. A magnetic field sensor is positioned to sense axialmagnetic field strength at a location offset from the second axis.Importantly, the magnetization of the second magnet gives rise to asensed magnetic field at the sensor that is nonsinusoidal with respectto an angle of rotation of the second magnet on the second axis, orgives rise to north and south poles separated by more than a half-circleor axial rotation of the second magnet.

BRIEF DESCRIPTION OF DRAWINGS

[0007] The invention will be described with respect to a drawing inseveral figures, of which:

[0008]FIG. 1 shows a prior art sensing system employing a radiallymagnetized magnet and a radial sensor;

[0009]FIG. 2 shows a prior art sensing system employing a radiallymagnetized magnet and an axial sensor;

[0010]FIGS. 3a and 3 b show axial and radial views, respectively, of asensing system such as that of FIG. 2;

[0011]FIG. 4 shows a shaped top magnet according to the invention;

[0012]FIG. 5 shows a gauge according to the invention employing anintentionally nonlinear dial deflection;

[0013]FIG. 6 shows a plot of magnetic field strength (e.g. gauss) as afunction of magnet angle for a round magnet radially magnetized;

[0014]FIG. 7 shows a plot of Hall-effect sensed voltage from an axiallypositioned sensor relative to a radially magnetized magnet, assuming avoltage offset so that all electrical outputs are positive;

[0015]FIG. 8 shows a plot of Hall-effect sensed voltage from an axiallypositioned sensor relative to a rotating magnet, assuming a symmetricmagnetization, or assuming a magnetization selected to give rise to alinear sensed voltage;

[0016]FIG. 9 shows a plot of magnetic field strength (e.g. gauss) as afunction of air gap between an axial sensor and a magnet radiallymagnetized;

[0017]FIG. 10 shows a plot of expected magnetic field strength (e.g.gauss) using an axial sensor and a magnet radially magnetized, as afunction of magnet angle, for each of three different choices of airgap;

[0018]FIG. 11 shows an experimentally determined plot of magnet heightat various magnet angles, according to the invention, giving rise to alinear or nearly linear output as a function of magnet angle;

[0019]FIG. 12 shows an experimentally measured plot of magnetic fieldstrength (e.g. gauss) as a function of magnet angle, according to theinvention, using the magnet heights of FIG. 11;

[0020]FIG. 13 shows an experimentally determined plot of magnet heightat various magnet angles, according to the invention, giving rise to anintentionally nonlinear output as a function of magnet angle;

[0021]FIG. 14 shows an experimentally measured plot of magnetic fieldstrength (e.g. gauss) as a function of magnet angle, according to theinvention, using the magnet heights of FIG. 14;

[0022]FIG. 15 shows an experimentally measured plot of Hall-effectoutput voltage as a function of dial reading, according to theinvention, which is expected to correlate linearly with liquid level;

[0023]FIG. 16 shows a typical arrangement of float, float arm, pivot,linkage, and magnet juxtapositions, as found in the prior art and in thesystem according to the invention;

[0024]FIG. 17 shows a cross section of a prior art magnet;

[0025]FIG. 18 shows a cross section of a shaped-top magnet;

[0026]FIG. 19 shows a cross section of a balanced shaped-top magnet;

[0027]FIG. 20 shows a shaped-top magnet positioned by means of a shaft;

[0028]FIG. 21 shows a shaped-top magnet in combination with a flux pathelement nearly completing a flux loop;

[0029]FIG. 22 shows the combination of FIG. 21 together withrepresentative lines of magnetic flux;

[0030]FIG. 23 shows a top view of the combination of FIG. 21;

[0031]FIG. 24 shows the combination of FIG. 23 together withrepresentative lines of magnetic flux;

[0032]FIG. 25 shows a float arrangement in greater detail than FIG. 16;and

[0033]FIG. 26 shows the float arrangement of FIG. 26 in partial cutawayview.

DETAILED DESCRIPTION

[0034] Turning first to FIG. 16, what is shown is a typical arrangementof float 80, float arm 81, pivot 82, linkage 83, and magnetjuxtapositions, as found in the prior art and in the system according tothe invention. The float 80 is caused to move upwards and downwards bythe downward pull of gravity and by the upward force from the level ofthe liquid 86. Float 80 is selected to be less dense, and preferablymuch less dense, than the liquid 86. The upward force from the liquidis, of course, a function of the mass of the volume of liquid displacedby the portion of the float that is below the surface of the liquid. Thefloat arm 81 and the rest of the internal parts are selected to haveminimal volume so as to minimize the portion of the volume of thecontainer 87 that is unavailable for liquid storage due to the presenceof the sensing apparatus. The system is particularly helpful in caseswhere the liquid 86 is flammable, such as liquified natural gas,liquified propane, gasoline, jet fuel, or diesel fuel. Where the liquidhas a relatively high vapor pressure (e.g. liquified natural gas orliquified propane) the container 87 is sealed (in part by divider orpartition 84) and the gas phase and liquid phase of the stored fuel arein equilibrium determined by temperature and other factors. The divideror partition 84 is of course preferably nonmagnetic, so as to permitpassage of magnetic flux; in a typical embodiment the divider isaluminum.

[0035] A mechanical linkage is provided which, in an exemplaryembodiment, translates rotation of the arm 81 about the pivot 82 intorotation of a shaft 83. Shaft 83 causes magnet 88 to rotate. Despite thepresence of the partition 84, the magnet 88 is magnetically linked withthe magnet 21. Preferably the two magnets 88, 21 rotate coaxially.Sensor 85 senses the position of the magnet 21. The magnet 21 preferablyhas a radial pointer, omitted for clarity from FIG. 16, which is visibleto a human user, also omitted for clarity from FIG. 16. The radialpointer can point to scale markings permitting the human user to readthe liquid level in the container 87. The scale preferably showspercentage of capacity but may also read in units such as mass or weightor volume of liquid.

[0036] Turning now to FIGS. 25 and 26, a typical float arrangement isshown in greater detail. In FIG. 25, liquid level gauge 120 is shown,part of which is gauge head 122.

[0037] Support arm 124 extends away from head 122. A magnet driveshaft126 communicates between gears 136, 134 and a first magnet, not shown inFIG. 25.

[0038] Arm 130 rotates about pivot axis 138, carrying float 132. Dialassembly 160 provides angle measurement.

[0039] In FIG. 26, more detail can be seen of the dial assembly 160.Gauge head 122 is mounted to a tank (not shown) by threads 142. Shaft126 has a first end 141 which is attached to magnet 140. Magnet 154follows magnet 140. Magnetic sensor 158 detects the field from magnet154. Passageway 144 contains magnet 140. Shaft 124 has an end 143 whichis mounted to the head 122. The magnet 154 preferably rotates on a pin152, which may be integrally formed with other plastic parts such asbase 148 and preferably clear plastic cover 156. Electrical leads 160extend from sensor 158. Base 1148 fits into receptacle 146.

[0040] It must be appreciated that while FIGS. 16,25, and 26 showpreferred mechanical arrangements, these particular arrangements aremerely considered preferable but are not required to obtain the benefitsof the invention.

[0041]FIG. 1 shows a prior art sensing system employing a radiallymagnetized magnet 21 and a radial sensor 22. In such a system the sensor22 is positioned radially from the magnet 21. The magnet 21 has amagnetization shown symbolically by arrow 26, which magnetization isradial and not axial. Those skilled in the art will appreciate that themagnetization might merely have a component in the plane parallel to theaxis 27 (coming out of the page) to bring about a measurable signal atthe sensor 22, though it is preferable that the magnetization be whollywithin the plane parallel to the axis 27. This magnetization is stylizedby north pole 24 and south pole 25. Electrical conductors 23 provideground, power, and sensed signal connections in a preferred embodimentusing a three-terminal Hall-effect sensor.

[0042]FIG. 2 shows a prior art sensing system employing a radiallymagnetized magnet 21 and an axial sensor 28. In such a system the sensor28 is positioned axially from the magnet 21, and its position is not inthe axis 27 but is radially offset from the axis 27. It is desirablethat the sensor be radially offset as far as possible so as to pick upthe strongest magnetic signals and thus to have the best possiblesignal-to-noise ratio.

[0043]FIGS. 3a and 3 b show axial and radial views, respectively, of aprior art sensing system such as that of FIG. 2. FIG. 3a correspondsclosely with FIG. 2. FIG. 3b shows an air gap 29 which exists because ofthe nonzero distance between the sensor 28 and the surface of the magnet21. The distance is desirably nonzero because it is harmful to havemechanical interference (e.g. friction) which might keep the magnet 21from freely rotating to follow the magnet 88 (FIG. 16).

[0044] Turning now to FIG. 6, what is shown is a plot of magnetic fieldstrength (e.g. gauss) as a function of magnet angle for a round magnet21 (FIG. 3a) radially magnetized. The curve is, as expected from theory,essentially sinusoidal. The voltage output from the sensor 28 (FIG. 3a)is portrayed in FIG. 7, which shows a plot of Hall-effect sensed voltagefrom the axially positioned sensor 28 relative to the radiallymagnetized magnet 21. In this plot, it is assumed that a voltage offsetis applied to the sensed voltage signal so that all electrical outputsare positive. With such an offset, the voltage in this case swingsbetween zero and 5 volts, about a center value of 2.5 volts. Thecorrespondence between FIGS. 6 and 7 is a consequence not only of thisvoltage offset but also of a gain assumed to be five millivolts perGauss. As expected from theory, this curve is also sinusoidal.

[0045] The curve of FIG. 7 illustrates a classic “disambiguation”problem. Suppose the sensed voltage is 1.0 volt. Such a voltage could,on the assumptions underlying FIG. 7, result from a magnet angle ofabout 35 degrees or about 145 degrees. The precise angles are a functionof gains and offsets and system geometries. With different gains andangles and geometries the particular angles corresponding to 1.0 voltmight be different, but in any event there would be an ambiguity—is thesensor at one angle or at a different angle? There are several ways ofdealing with (or avoiding) this ambiguity. One choice is to imposemechanical constraints so that the magnet angle never gets below 90degrees or above 270 degrees. In fact to accommodate various errorranges, the constraint must impose a margin, so that the angle is neverpermitted to get below 90+x or above 270−x, where x is some positivevalue determined by experimentally determined error ranges. Returning toFIG. 16, this may require careful selection of the gear ratio betweenthe shaft 83 and the arm 81. If the magnet 21 carries a human-readablepointer, the pointer is constrained to a range of something less than180 degrees, making the gauge harder to read because the full-to-emptymarkings must fit into less than 180 degrees. Such angle restrictionalso limits the accuracy of the electrical measurements because even asmall angle error (in absolute terms) gets magnified into a change ofsome number of bits after the A/D conversion.

[0046] A second way of dealing with this ambiguity is to sample theangle sufficiently frequently that one can be confident of the recentangle position as well as the recent rate of change of the angle(angular velocity). This can permit determining whether the magnet is at35 degrees or 145 degrees from detailed knowledge of the previousposition and velocity. This approach offers many drawbacks, of course.The system must maintain internal states such as historical position andvelocity data. In addition, it must provide and allocate computationalbandwidth for the necessary frequent sampling of the sensor data.Finally, there is the problem that if power is lost, the float may moveduring the outage and upon restoration of power it may be impossible toresolve initial ambiguity in the magnet angle.

[0047] Yet another problem evident from the curve of FIG. 7 is thenonlinearity problem. Even if the magnet angle is not permitted to straybeyond 90+x or 270−x, within that range the relationship between voltageand angle is not linear. Depending on the linkage between the float(FIG. 16) and the magnet 88 (FIG. 16), further nonlinearities may beintroduced in the relationship between liquid level and voltage. Whilesuch nonlinearities may be corrected later in software, this may add tothe hardware cost (by requiring more computational power) and will leadto non-constant resolution in differing parts of the measurement rangesince differing computational gains must be applied in differing partsof the range for the correction.

[0048]FIG. 9 shows an experimentally measured plot of magnetic fieldstrength (e.g. gauss) as a function of air gap between an axial sensorand a magnet radially magnetized. Referring to FIG. 3b, increasing theair gap 29 decreases the detected magnetic field strength.

[0049]FIG. 10 shows a plot of expected magnetic field strength (e.g.gauss) using an axial sensor and a magnet radially magnetized. The fieldstrength is shown as a function of magnet angle, for each of threedifferent choices of air gap. Curve 50 shows the measured field strengthwith a close (small) air gap, curve 51 shows the result with anincreased air gap, and curve 52 shows the result with an even larger airgap. For one skilled in the art, motivated to attempt to solve theproblems discussed here, the dependence of sensed flux on the size ofthe air gap might prompt any of several approaches. For example, itmight be considered to attempt to use an axial cam arrangement tophysically move the rotating magnet closer to and further from thesensor as a function of rotation angle, or to use an axial camarrangement to physically move the sensor closer to or further from themagnet as a function of rotation angle. Such approaches, however, offerseveral drawbacks. One drawback is that the cam arrangement likelyintroduces friction which can lead to lags in the response of the magnet21 to small rotations of the magnet 88. Yet another is an increase inparts count and thus assembly time and cost, as well as a likelydecrease in reliability.

[0050] Insight may be drawn from FIGS. 9 and 10, to devise a magnet thathas a shaped top. Resulting from such insight, FIG. 4 shows a shaped-topmagnet 31 according to the invention. The magnet 31 offers a low point33, roughly midway between the north and south poles 24, 25. The magnet31 offers high points 34 roughly opposite from the low point 33. Themagnet 31 also preferably has a pointer 32 providing a human-readableindication of the magnet angle. Optionally the shaped-top magnet maylocate the north and south poles at an angular relationship that is not180 degrees apart. For example the north and south poles might be up to300 degrees apart (or, completing the circle, 60 degrees apart).

[0051]FIG. 8 shows a plot of Hall-effect sensed voltage from an axiallypositioned sensor relative to a rotating magnet, assuming a symmetricmagnetization and assuming a flat top, or assuming a magnetization andtop shape selected to give rise to a linear sensed voltage. The axes ofFIG. 8 carry the same units as the axes of FIG. 7. The vertical scale isHall-effect sensed voltage, taking into account the same offsetdiscussed above in connection with FIG. 7, so that the range of outputvoltages is always positive with a center of travel at about 2.5 volts.Dotted line 41 shows the expected sinusoidal relationship between angleand measured voltage, just as in FIG. 7, on the assumption that themagnet has a flat (not shaped) top and has north and south poles 180degrees apart.

[0052] With appropriate placement of the north and south poles, and withappropriate shaping of the top of the magnet, very desirable results maybe obtained. Curve 43 (actually a straight line) offers a relationshipbetween measured voltage and magnet angle that fulfills two importantand previously unattained conditions—the relationship between voltageand angle are linear, and the physically measurable range is from 30degrees to 330 degrees. This range, covering 300 degrees of rotationwithout any ambiguities as would be found with the curve of line 41, ismuch better than the somewhat less than 180 degrees available with thecurve of line 41.

[0053]44 is intentionally nonlinear when compared with line 43. It mustbe recalled that the actual design goal is not linearity between magnetangle and voltage, but linearity between liquid level and voltage. Thefloat linkage (for example that of FIG. 16) is itself somewhat nonlinearin the relationship between liquid level and magnet angle. Appropriatechoices of magnet top shape (FIG. 4) can bring about intentionalnonlinearity between magnet angle and voltage that bring about anoverall linear relationship between liquid level and voltage.

[0054] Those skilled in the art will appreciate that there may be othersources of nonlinearity in the relationship between liquid level andmagnet angle. The container 87 might have a non-constant cross sectionas a function of the liquid level. This could happen if, for example,the container were spherical instead of cylindrical. Even if thecontainer 87 were chiefly cylindrical, it might have a hemisphericalbottom (as in a small hand-carried LP tank) or might be a lateralcylinder with hemispherical ends (as in a residential heating fuel LPtank). In any of these cases, the magnet top is advantageously shaped togive rise to overall linearity between the amount of liquid present andthe measured voltage.

[0055] Those skilled in the art will readily appreciate that thebenefits of the invention do not depend on a particular mechanicallinkage such as that shown in FIG. 16. Indeed those skilled in the artwill have no difficulty devising other mechanical linkages and floatarrangements that likewise benefit from the invention. For example, theshaft 83 could be vertical instead of horizontal, with the magnet 88rotating at the top of the shaft in a horizontal plane. The shaft 83could then be threaded with a partial thread, with the float freelysliding up and down the shaft, causing the shaft to rotate to an angledetermined by the height of the float. Appropriate selection of magnettop shape can give overall linearity for other mechanical linkages andfloat arrangements.

[0056] Still another possibility is that the physical phenomenon beingsensed is not communicated with a shaft but is instead communicated bylinkage with a crank arm.

[0057]FIG. 5 shows a gauge according to the invention employing anintentionally nonlinear dial deflection. Pointer 32 is visible, and themarkings from E (empty) to full span well over 180 degrees. A clearcover protects the magnet 31 and permits a user to see the pointer.Connection points 23 provide connections to the Hall-effect sensor, notvisible in FIG. 5.

[0058]FIG. 11 shows an experimentally determined plot 53 of magnetheight at various magnet angles, according to the invention, giving riseto a linear or nearly linear output as a function of magnet angle. If itis desired to have a linear relationship between magnet angle andvoltage, then it is advantageous to use a shaped top magnet with the topheights chosen according to FIG. 11.

[0059] One way to describe the plot 53, and to describe the shaped-topmagnet defined by the plot 53, is to say that the magnet has a faceproximal to a magnetic field sensor, the face of the magnet shaped toprovide a varying gap between the face of the magnet and the magneticfield sensor as a function of an angle of rotation of the magnet on itsaxis, the shape of the face selected to provide a first gap at a firstangular position, a second gap larger than the first gap at a secondangular position, and a third gap smaller than the second gap at a thirdangular position. The difference between the first and third angularpositions may be greater than 200 degrees, and preferably may be greaterthan 250 degrees, and indeed may be greater than 280 degrees.

[0060] Those skilled in the art will appreciate that different materialswill produce different flux outputs as a function of the size of the airgap, and that different physical shapes may be needed depending on thechoice of material. An exemplary material for the shaped top magnet isferrite molded with a polymer binder material to the desired shape, andthen magnetized.

[0061]FIG. 12 shows an experimentally measured plot of magnetic fieldstrength (e.g. gauss) as a function of magnet angle, according to theinvention, using the magnet heights of FIG. 11. The curve 54 shows goodlinearity from less than 50 degrees to more than 310 degrees. This isconsistent with the linear plot 43 in FIG. 8, differing in that FIG. 8shows voltage as a function of angle, whereas FIG. 12 shows magneticfield strength as a function of angle. FIG. 8 also assumes an offset sothat all voltages are positive, whereas the field strengths of FIG. 12range from positive to negative.

[0062] mentioned above, the system goal is usually not linearity betweenmagnet angle and voltage, but linearity between liquid level andvoltage. As such, the relationship between magnet angle and voltageshould be intentionally nonlinear in a way that corrects fornonlinearity between liquid level and magnet angle. Turning now to FIG.14, it might develop that the desired nonlinearity is that shown incurve 54 of FIG. 14. FIG. 13, then, shows an experimentally determinedplot 55 of magnet height at various magnet angles, according to theinvention, giving rise to an intentionally nonlinear output as afunction of magnet angle as shown in FIG. 14.

[0063] As discussed, the nonlinearity of the float and linkage ispreferably compensated by a correcting nonlinearity in the relationshipbetween magnet angle and voltage. FIG. 15 shows an experimentallymeasured plot 57 of Hall-effect output voltage as a function of dialreading, according to the invention. As may be appreciated, therelationship is nearly linear across almost all of the range from 10% to90%. With appropriate choices of float and linkage, the measurementrange could extend from 0% to 100%. With many real-life applications,however, such as knowing when to refill a tank, it suffices to measurevalues between 10% and 90%.

[0064] Those skilled in the art are familiar with standard, prior-artHall-effect sensors which have integral temperature compensationcircuitry. In particular, the above-mentioned three-terminal Hall-effectsensors are available as standard parts with temperature compensationcircuitry. Some such parts are programmable by the user to defineparticular temperature compensation behavior. The obvious use of theprogrammability of the temperature compensation circuitry is to achievea nearly constant output of the sensor, as a function of sensed magneticflux or sensed physical position, despite changes in temperature.

[0065] As mentioned above, the equilibrium between liquid and gas phasesin a liquified propane or liquified natural gas storage tank is afunction of several factors including temperature. For safety, it isdesired never to fill the tank fully with liquid, but always to leave aportion of the tank for the gas phase. One safety goal is to avoid anexcessive level of gas pressure in the tank. The portion of the tank tobe left for the gas phase for safety reasons is, itself, a function oftemperature. Those skilled in the art will thus appreciate that thequestion “is the tank full?” is not exactly the same as “what is theliquid level in the tank?” When the temperature is low, the vaporpressure decreases, and the equilibrium in the tank tends to shifttoward the liquid phase. It is desirable not to fill the tank as fullyif the temperature is low, because later if the temperature increasesthe vapor pressure would increase and the equilibrium would shiftsomewhat toward the gas phase.

[0066] In accordance with the invention, it is thus possible to programthe temperature compensation circuitry of the Hall-effect sensor so asto take into account (at least partially) the effect of temperature onthe stored liquified gas. In a typical application of this aspect of theinvention, the temperature compensation circuitry of the Hall-effectsensor is programmed so that the electrical signal indicative of a“full” tank is generated by any of several levels of sensed magneticflux, depending on temperature. If it is cold, the amount of sensed fluxneeded to generate a “full” electrical output is less then if it iswarm, for example. The programming of the temperature compensationcircuitry, together with appropriate selection of magnet shape and othersystem geometries, can thus achieve an approximation of the full orempty status of the storage enclosure that is more useful than merelydetecting a liquid level.

[0067] This desirable result which takes temperature into account mayalso be accomplished by providing a temperature sensor separate from themagnetic field sensor but housed nearby to it, with appropriatecircuitry to take temperature into account before the signal for lines23 is generated.

[0068] Stated differently, the sensing system may include a temperaturesensor, the sensing system further characterized as being used with aliquid having a vapor pressure and a container having a geometry andvolume, the magnetic field sensor emitting a signal indicative of sensedmagnetic field strength, the system further comprising compensationmeans compensating the signal with respect to the sensed temperature andthe liquid having a vapor pressure and with respect to the containerhaving a geometry and volume to yield the electrical output, whereby theelectrical output is indicative of the fullness of the container.

[0069]17 shows a side cross section of a magnet 21 according to theprior art. An axial hole 97, centered in the bottom face of the magnet21, is formed so that the magnet can rotate upon a stationary spindle,omitted for clarity in FIG. 17. A feature 96 formed onto the top of themagnet 21 preferably defines a mechanical tolerance for axial movementof the magnet 21 within its housing. In the system according to theinvention, as described above, the magnet 31 has a shaped top, and partof the shaping of the top may be perceived in FIG. 18 in the slope thatis downward to the right at the top of the magnet 31.

[0070] A full consideration of the forces acting upon the magnet 21 or31 would take into account not only the magnetical dipole of magnet 88,but also gravity. If the axis of rotation of magnet 21 or 31 isvertical, then gravity will not tend to cause rotation of the magnet 21,31. In some applications, however, the gauging system may be positionedso that the axis of rotation is non-vertical. In such applications, itmay be appreciated that the magnet 31 may have a center of mass that isnot on-axis. The center of mass is shifted by the mass of the pointer 32and by the shaping of the top of the magnet 31. In FIG. 19, for example,the center of mass is to the left of the center hole 97. Thus, accordingto the invention, it is desirable to provide a relief area 98. Withappropriate selection of the relief area 98, the center of mass can beshifted back toward the axis defined by center hole 97. In this way, theangle of the magnet 31 is substantially unaffected by gravity and thusprovides more accurate readings. Stated differently, the magnet 31 willtrack the magnet 88 more faithfully because it is gravitationallybalanced.

[0071] While the inventive benefits of the shaped-top magnet have beendescribed in detail above in a system where the magnet 31 rotates due tomagnetic coupling to another magnet 88, it should be appreciated thatthese same benefits may be enjoyed in systems employing other couplings.As shown in FIG. 20, a shaped-top magnet 31 may be caused to rotatebecause of a direct mechanical connection with an axle 99. Rotation ofthe axle 99 causes a rotation of shaped-top magnet 31, the angle ofwhich is sensed by means of sensor 28 with electrical leads 23. Thepointer 32 may or may not be needed, depending on whether there is aneed for a user to be able to observe the position, for example on ascale.

[0072] another possibility is that the physical phenomenon being sensedis not communicated with a shaft or axle 99 but is instead communicatedby linkage with a crank arm or some other mechanical linkage.

[0073] Turning now to FIG. 21, what is shown is a shaped-top magnet incombination with a flux path element nearly completing a flux loop.Magnet 31 has optional pointer 32 as described above. Magnet 31 iscaused to rotate, either by magnetic linkage to a second magnet such asdescribed above, or by mechanical linkage such as to shaft 99. A washer77 may be fixed to the magnet 31 and thus rotates with the magnet 31.Magnetic sensor 28 is positioned between the washer 77 and the magnet31.

[0074]FIG. 22 shows the combination of FIG. 21 together withrepresentative lines of magnetic flux 78, 79. Importantly, the lines offlux 79 are fairly close to being parallel, which is a rather differentsituation than would obtain in the absence of washer 77. In the absenceof washer 77, the lines of flux 79 would likely diverge like the lines78. Such divergence leads to loss of accuracy if the magnet 31 movesaxially, as might happen in some situations, for example with wear inaxial bearings on the shaft 99. The presence of the washer 77, made of amaterial selected for its ability to provide an easy flux path, leads tothe fairly parallel lines 79. Lines 79 may indeed develop to be not onlyparallel but axial, all of which contributes to the result that readingsat the sensor 28 are fairly consistent even with axial movement of themagnet 31.

[0075] It should be appreciated that the arrangement with the washer 77fixed to the magnet 31 is thought to be preferable, but some of thebenefits of the invention would also be available if the washer werefixed to a housing and did not rotate with the magnet 31.

[0076]FIG. 23 shows a top view of the combination of FIG. 21. Theoptional pointer 32 may be seen, as well as the washer 77. Shaft 99 andsensor 28 are shown in phantom. FIG. 24 shows the combination of FIG. 23together with representative lines of magnetic flux 76. As may be seen,a typical configuration has north and south poles as shown in FIG. 24.

[0077] pointer 32 may be formed with the magnet 31, but may also beformed with the washer 77 or may be some non-magnetic material attachedto the washer 77. Alternatively, washer 77 may bear a scale which isread relative to a fixed pointer attached to a housing, not shown inFIG. 24.

[0078] The flux path element is shown as a washer 77, but it should beappreciated that its benefits are available even if the washer is notfully circular. For example, the flux path element could be asemicircular disk positioned so that it covers the range of movementabout the sensor. It can be of iron or magnetic steel or could beintegrally formed with the magnet 31 and from the same material as themagnet 31.

[0079] It should also be appreciated that the system of two linkedmagnets offers its benefits even if the physical quantity being sensedis not a liquid level measured with a float. The separation between thetwo magnets can separate any two environments, one of which might be anenvironment of a hazardous or flammable gas, or might be under verydifferent air pressure, for example.

[0080] It should likewise be appreciated that the linearization approachdescribed here offers its benefits even if there is no need to extendrange past 180 degrees.

[0081] Those skilled in the art will have no difficulty identifying anddevising myriad obvious variations of the invention without departing inany way from the invention, as defined by the claims which follow.

1. A liquid level sensing system for use with a container and protrudingthrough an opening in the container, the sensing system comprising: afloat; a first magnet, said first magnet axially rotatable on a firstaxis, said first magnet having a magnetic moment with a nonzerocomponent at a right angle to the axis; a linkage between the float andthe first magnet causing the first magnet to rotate in response tomovement of the float; a second magnet having a magnetization, saidsecond magnet axially rotatable on a second axis, said magnetization ofthe second magnet giving rise to a nonzero component at a right angle tothe second axis, the first and second magnets juxtaposed in magneticlinkage so that the second magnet is urged to follow the first magnet inrotation; a divider separating the first and second magnets, the dividerplugging the opening in the container; a magnetic field sensorpositioned to sense axial magnetic field strength at a location offsetfrom the second axis; the second magnet having a face proximal to themagnetic field sensor, the second magnet giving rise to a sensedmagnetic field at the magnetic field sensor, the axial face of thesecond magnet shaped to provide a varying gap between the face of thesecond magnet and the magnetic field sensor as a function of an angle ofrotation of the second magnet on the second axis, the shape of the faceselected to provide a first gap at a first angular position, a secondgap larger than the first gap at a second angular position, and a thirdgap smaller than the second gap at a third angular position, thedifference between the first and third angular positions being greaterthan 200 degrees, the second angular position being between the firstand third angular positions.
 2. The sensing system of claim 1 whereinthe magnetic field sensor is a Hall-effect sensor.
 3. The sensing systemof claim 1 wherein the difference is greater than 250 degrees.
 4. Thesensing system of claim 1 wherein the difference is greater than 280degrees.
 5. The sensing system of claim 1 wherein the second magnet isfurther characterized in that its center of mass is substantially on thesecond axis.
 6. The sensing system of claim 1 wherein the second magnetfurther comprises a pointer, the pointer visible to a user.
 7. Thesensing system of claim 6 wherein the second magnet is furthercharacterized in that its center of mass is substantially on the secondaxis.
 8. The sensing system of claim 1 further comprising an electricaloutput and a temperature sensor, the sensing system furthercharacterized as being used with a liquid having a vapor pressure and acontainer having a geometry and volume, the magnetic field sensoremitting a signal indicative of sensed magnetic field strength, thesystem further comprising compensation means compensating the signalwith respect to the sensed temperature and the liquid having a vaporpressure and with respect to the container having a geometry and volumeto yield the electrical output, whereby the electrical output isindicative of the fullness of the container.
 9. The sensing system ofclaim 1 further comprising a flux path element positioned relative tothe second magnet with the magnetic field sensor between the secondmagnet and the flux path element.
 10. A liquid level sensing system foruse with a container and protruding through an opening in the container,the sensing system comprising: a float; a first magnet, said firstmagnet axially rotatable on a first axis, said first magnet having amagnetic moment with a nonzero component at a right angle to the axis; alinkage between the float and the first magnet causing the first magnetto rotate in response to movement of the float; a second magnet having amagnetization, said second magnet axially rotatable on a second axis,said magnetization of the second magnet giving rise to a nonzerocomponent at a right angle to the second axis, the first and secondmagnets juxtaposed in magnetic linkage so that the second magnet isurged to follow the first magnet in rotation; a divider separating thefirst and second magnets, the divider plugging the opening in thecontainer; a magnetic field sensor positioned to sense axial magneticfield strength at a location offset from the second axis; themagnetization of the second magnet giving rise to north and south polesseparated by more than 200 degrees of axial rotation of the secondmagnet on the second axis.
 11. The sensing system of claim 10 whereinthe magnetic field sensor is a Hall-effect sensor.
 12. The sensingsystem of claim 10 wherein the separation of the north and south polesof the second magnet exceeds 250 degrees.
 13. The sensing system ofclaim 12 wherein the separation of the north and south poles of thesecond magnet exceeds 280 degrees.
 14. The sensing system of claim 10further comprising a flux path element positioned relative to the secondmagnet with the magnetic field sensor between the second magnet and theflux path element.
 15. An angle sensing system comprising: a magnethaving a magnetization, said magnet axially rotatable on an axis, saidmagnetization of the magnet giving rise to a nonzero component at aright angle to the axis; a magnetic field sensor positioned to senseaxial magnetic field strength at a location offset from the axis; themagnet having a face proximal to the magnetic field sensor, the magnetgiving rise to a sensed magnetic field at the magnetic field sensor, theface of the magnet shaped to provide a varying gap between the face ofthe magnet and the magnetic field sensor as a function of an angle ofrotation of the magnet on the axis, the shape of the face selected toprovide a first gap at a first angular position, a second gap largerthan the first gap at a second angular position, and a third gap smallerthan the second gap at a third angular position, the difference betweenthe first and third angular positions being greater than 200 degrees,the second angular position being between the first and third angularpositions.
 16. The sensing system of claim 15 wherein the magnetic fieldsensor is a Hall-effect sensor.
 17. The sensing system of claim 15wherein the difference is greater than 250 degrees.
 18. The sensingsystem of claim 17 wherein the difference is greater than 280 degrees.19. The sensing system of claim 15 wherein the magnet further comprisesa pointer, the pointer visible to a user.
 20. The sensing system ofclaim 15 further comprising a flux path element positioned relative tothe magnet with the magnetic field sensor between the magnet and theflux path element.
 21. A sensing system for use with a boundary andprotruding through an opening in the boundary, the sensing systemcomprising: a first magnet, said first magnet axially rotatable on afirst axis, said first magnet having a magnetic moment with a nonzerocomponent at a right angle to the axis; a linkage between a sensedphenomenon and the first magnet causing the first magnet to rotate inresponse to the sensed phenomenon; a second magnet having amagnetization, said second magnet axially rotatable on a second axis,said magnetization of the second magnet giving rise to a nonzerocomponent at a right angle to the second axis, the first and secondmagnets juxtaposed in magnetic linkage so that the second magnet isurged to follow the first magnet in rotation; a divider separating thefirst and second magnets, the divider plugging the opening in theboundary; a magnetic field sensor positioned to sense axial magneticfield strength at a location offset from the second axis; the secondmagnet having a face proximal to the magnetic field sensor, the secondmagnet giving rise to a sensed magnetic field at the magnetic fieldsensor, the axial face of the second magnet shaped to provide a varyinggap between the face of the second magnet and the magnetic field sensoras a function of an angle of rotation of the second magnet on the secondaxis, the shape of the face selected to provide a first gap at a firstangular position, a second gap larger than the first gap at a secondangular position, and a third gap smaller than the second gap at a thirdangular position, the difference between the first and third angularpositions being greater than 200 degrees, the second angular positionbeing between the first and third angular positions.
 22. The sensingsystem of claim 21 wherein the magnetic field sensor is a Hall-effectsensor.
 23. The sensing system of claim 21 wherein the difference isgreater than 250 degrees.
 24. The sensing system of claim 23 wherein thedifference is greater than 280 degrees.
 25. The sensing system of claim21 wherein the second magnet is further characterized in that its centerof mass is substantially on the second axis.
 26. The sensing system ofclaim 21 wherein the second magnet further comprises a pointer, thepointer visible to a user.
 27. The sensing system of claim 26 whereinthe second magnet is further characterized in that its center of mass issubstantially on the second axis.
 28. A sensing system for use with aboundary and protruding through an opening in the boundary, the sensingsystem comprising: a first magnet, said first magnet axially rotatableon a first axis, said first magnet having a magnetic moment with anonzero component at a right angle to the axis; a linkage between asensed phenomenon and the first magnet causing the first magnet torotate in response to the sensed phenomenon; second magnet having amagnetization, said second magnet axially rotatable on a second axis,said magnetization of the second magnet giving rise to a nonzerocomponent at a right angle to the second axis, the first and secondmagnets juxtaposed in magnetic linkage so that the second magnet isurged to follow the first magnet in rotation; a divider separating thefirst and second magnets, the divider plugging the opening in theboundary; a magnetic field sensor positioned to sense axial magneticfield strength at a location offset from the second axis; themagnetization of the second magnet giving rise to north and south polesseparated by more than 200 degrees of axial rotation of the secondmagnet on the second axis.
 29. The sensing system of claim 28 whereinthe magnetic field sensor is a Hall-effect sensor.
 30. The sensingsystem of claim 28 wherein the separation of the north and south polesof the second magnet exceeds 250 degrees.
 31. The sensing system ofclaim 28 wherein the separation of the north and south poles of thesecond magnet exceeds 280 degrees.
 32. The sensing system of claim 28further comprising a flux path element positioned relative to the secondmagnet with the magnetic field sensor between the second magnet and theflux path element.
 33. An angle sensing system comprising: a magnethaving a magnetization, said magnet axially rotatable on an axis, saidmagnetization of the magnet giving rise to a nonzero component at aright angle to the axis;magnetic field sensor positioned to sense axialmagnetic field strength at a location offset from the axis; the magnethaving a face proximal to the magnetic field sensor, the magnet givingrise to a sensed magnetic field at the magnetic field sensor, the faceof the magnet shaped to provide a varying gap between the face of themagnet and the magnetic field sensor as a function of an angle ofrotation of the magnet on the axis, the shape of the face selected toprovide a first gap at a first angular position, a second gap largerthan the first gap at a second angular position, and a third gap smallerthan the second gap at a third angular position, the second angularposition being between the first and third angular positions.
 34. Thesensing system of claim 33 wherein the magnetic field sensor is aHall-effect sensor.
 35. The sensing system of claim 33 wherein thedifference between the first and third angular positions is greater than200 degrees.
 36. The sensing system of claim 35 wherein the differenceis greater than 250 degrees.
 37. The sensing system of claim 36 whereinthe difference is greater than 280 degrees.
 38. The sensing system ofclaim 33 wherein the magnet further comprises a pointer, the pointervisible to a user.
 39. The sensing system of claim 33 further comprisinga flux path element positioned relative to the second magnet with themagnetic field sensor between the second magnet and the flux pathelement.